1. Field of the Invention
The present invention relates to the reduction of NO.sub.x from engine exhaust emissions, and more particularly to the removal of NO.sub.x from diesel engine exhaust.
2. Description of Related Art
The control of NO.sub.x emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called "lean-burn" gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NO.sub.x removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NO.sub.x emissions under net oxidizing conditions is also critical for diesel engines.
According to a report published February 1992 by the U.S. Environmental Protection Agency, (Office of Air and Radiation, Office of Air Quality Planning and Standards, Research Triangle Park, N.C. 27711), there are, in general, four approaches to controlling NO.sub.x emissions from combustion sources. For example, controlling NO.sub.x formation by modifying the combustion operating conditions, by modifying the combustion equipment, by fuel switching, and by post combustion control of NO.sub.x by flue or exhaust gas treatment. The first three approaches reduce the original formation of NO.sub.x. The latter converts the NO.sub.x that was formed (in the exhaust gas) to something more benign.
With respect to lean-burn engines, catalysts (i.e., catalysts that can decompose NO.sub.x to N.sub.2 and O.sub.2 in oxygen-rich environments) that have the activity, durability, and temperature window required to effectively remove NO.sub.x from the exhaust have not been effective. Prior art lean-NO.sub.x catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges. Conventional catalysts are therefore inadequate for lean-burn operation and ordinary driving conditions. An alternative is to use catalysts that selectively reduce NO.sub.x in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant.
Using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline and diesel engines as a co-reductant is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon co-reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
SCR catalysts that selectively promote the reduction of NO.sub.x under oxygen-rich conditions in the presence of co-reductant hydrocarbons are known as lean-NO.sub.x catalysts. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, reductants, and conditions. The relatively expensive noble metal catalysts have exhibited high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. The excess oxygen adsorbs preferentially on the noble, precious metal, e.g., Pt, Rh, and Pd, surfaces in the catalyst, and inhibits a chemical reduction of NO.sub.x to N.sub.2 --instead promoting the oxidation of unburned hydrocarbons and carbon monoxide. This is because the CO and hydrocarbon reductants tend to react more quickly with the free oxygen, O.sub.2, present in the exhaust gas than the oxygen associated with nitrogen in NO.sub.x. Also, many lean-NO.sub.x catalysts are too susceptible to water vapor and high temperatures. As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. The deactivation is accelerated by the presence of water vapor in the stream. In addition, water vapor suppresses the NO reduction activity even at lower temperatures.
Thus, the problems encountered in lean-NO.sub.x catalysts include lessened activity of the catalyst in the presence of excessive amounts of oxygen (preference for oxidation of CO and hydrocarbons), reduced durability of the catalyst in the presence of water, sulfur, and high temperature exposure, and narrow temperature windows in which the catalyst is active. Practical lean-NO.sub.x catalysts must overcome all three problems simultaneously before they can be considered for commercial use.
Another major source of catalyst deactivation is high temperature exposure. This is especially true in automobile catalysts where temperatures close to 1000.degree. C. can exist. The high-temperatures attack both the catalyst precious metal and the catalyst carrier, e.g., gamma alumina (.gamma.-Al.sub.2 O.sub.3). Three-way catalysts, for instance, are comprised of about 0.1 to 0.15 percent precious metals on a .gamma.-Al.sub.2 O.sub.3 wash coat, and use La.sub.2 O.sub.3 and/or BaO for a thermally-stable, high surface area .gamma.-Al.sub.2 O.sub.3. Even though the precious metals in prior art catalysts were initially well dispersed on the .gamma.-Al.sub.2 O.sub.3 carrier, they were subject to significant sintering when exposed to high temperatures. This problem, in turn, led to the incorporation of certain rare earth oxides such as CeO.sub.2 to minimize the sintering rates of such precious metals.
In one high temperature application described in US. Pat. No. 5,618,505, issued to Subramanian et al., researchers have attempted to reduce NO.sub.x from internal combustion engine exhaust with relatively inexpensive base-metal-containing lean-NO.sub.x catalysts using a propane hydrocarbon coreductant. However, successful NO conversion percentages above 30 are only obtained with propane co-reductant at temperatures exceeding 450.degree. C. Such results are impractical for most, if not all, diesel internal combustion engine exhaust. Furthermore, tests of nine model fuels and a diesel fuel injected into an exhaust stream have shown no higher than 43% NO.sub.x conversions. See Collier and Wedekind, The Effect of Hydrocarbon Composition on Lean NO.sub.x Catalysts, SAE Technical Series 97300, Int. Fall Fuels & Lub Meeting & Expos., Tulsa, Okla., (October 1997).
The challenge still exists for lean-NO.sub.x catalysts promotion of NO.sub.x reduction at the lower combustion temperatures associated with diesel exhaust. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but no present solution exists for NO.sub.x.
Another existing challenge is to minimize the exhaust emission of the unused portion of added hydrocarbon vapor co-reductants during lean-NO.sub.x catalytic promotion of NO.sub.x reduction. Added hydrocarbon vapor coreductants can be directly injected into an exhaust stream in a controlled manner using flow controllers or vaporizing hydrocarbon liquids. Since utilization of the hydrocarbon vapor stream during catalytic reduction of NO.sub.x is not 100% efficient, unused hydrocarbon vapors escape from present processing systems. A need exists to maximize the reduction of NO.sub.x to N.sub.2 while minimizing the unused hydrocarbon vapor emission.